This invention relates to a magnetic clamping arrangement for holding the arrangement operably clamped with respect to a ferromagnetic surface and in particular relates to such clamping arrangement for releasably attaching a body with respect to a surface that is worn with small dimension imperfections, such as a pattern of grooves and/or ridges.
The invention is particularly applicable to the apparatus for measuring wear in the surface of a ferromagnetic rubbing member of a brake or like friction couple as described in WO99/17072.
Briefly, as a resume the above publication, wear and any wear pattern of a rubbing member, such as the ferromagnetic brake disc of a railway or like vehicle, is measured with the disc in situ on a wheel and in a vertical plane by periodically clamping to the disc surface a carrier of a scanning transducer probe that reciprocates along a predefined track and produces signals indicative of what wear has occurred.
To reposition the carrier member repeatedly with accuracy, indentations are made in the surface and the carrier aligned with these prior to being clamped to the surface by energising an electromagnet coupled to the carrier. The electromagnet supports the apparatus on such a vertical surface and is resiliently mounted to exert a bias on the carrier to ensure it remains located precisely.
Such a rubbing member is found to wear substantially uniformly in a so called coupled motion direction, that is, in the direction of relative motion between a friction material and the rubbing surface, or circumferentially of the disc, but to wear unevenly in a transverse, or radial, direction.
It has been found that in some such friction couples, the wear not only varies gradually across the rubbing surface, but also has superimposed thereon rapid changes in wear due primarily to the granular nature of the rubbing surface and/or friction material with its coarse mixture of materials of different hardness and abrasiveness, the rapid changes being of the form ridges or grooves and in the worst case, a succession thereof in the manner of a gramophone record.
In such circumstances it has been found difficult to guarantee a necessary degree of electromagnetic clamping between the electromagnet and the ferromagnetic surface due to the reduction in actual contact area between the magnet pole faces and the surface.
It will be appreciated that although this is a specific example of a situation wherein magnetic coupling is used to effect clamping of a carrier or other body to a ferromagnetic surface the need to clamp to such an uneven surface is not confined thereto, and without limiting the generality of the forgoing as to form and scale of unevenness, it is an object of the present invention to provide a magnetic clamping arrangement that is more able to accommodate local variation in surface profile without loss of efficacy than hitherto in a simple and cost effective manner.
According to the present invention a magnetic clamping arrangement for holding the arrangement operably clamped against a ferromagnetic surface comprises housing means supporting magnet means comprising a core having a plurality of limbs at least one of which comprises a plurality of elongate ferromagnetic elements, each having at least one dimension transversely to its length much smaller than its length, bundled together such that the ends of said elements define a pole face of the limb and are slidable with respect to each other in a direction to and from the pole face, to effect conforming abutment with a said ferromagnetic surface, flux generating means coupled to the core to cause a flux path between the pole faces and said ferromagnetic surface to effect an attachment force between the magnet means and surface, and element bias means, disposed between housing means and core, operable absent magnetic flux to define by the locus of the ends of the elements of each limb a quiescent pole face and exert a restoring force on elements of the bundle displaced away from the quiescent pole face, the arrangement being characterised by a body, having a datum face adapted to be disposed in overlying relationship with respect to said ferromagnetic surface, carrying the housing means movable with respect to the body in a direction to and from the datum face and attachment means, including resilient attachment bias means between the body and core arranged to bias the core elements towards a position in which the pole faces are recessed with respect to the body datum face, and attachment bias override means arranged to permit an override force to be exerted in opposition to the attachment bias to expose quiescent pole faces un-recessed with respect to the datum face for abutment with the ferromagnetic surface and in opposition to the element bias means to permit the elements in abutment with the ferromagnetic surface to conform thereto, the attachment bias means being operable, with the core pole faces attached to the ferromagnetic surface by said magnetic flux and absent said override force, to cause the datum face of the body to exert an abutting force against the ferromagnetic surface.
Such an arrangement permits individual core elements to be displaced relative to each other against the element bias in the quiescent state, that is, absent magnetic flux, to accommodate an uneven surface such that there is a greater area of intimate contact without air gaps. In the presence of the magnetic flux, forces exerted between adjacent elements of the bundle associated with any pole face effectively remove any tendency to be biassed back to the quiescent pole face locus.
The magnet may be of permanent magnet form but preferably is electromagnetic, including an electric coil surrounding at least part of the core.
The body may be disposed directly against the ferromagnetic surface or may overlie and clamp to the surface an intervening article which is shaped or apertured for the passage of the magnet means.
Conveniently, in such wear determining apparatus wherein the ferromagnetic surface is uneven due to parallel extending grooves and/or ridges, the elongate ferromagnetic elements comprise flat laminations, each lamination extending for the whole of one dimension of the pole face of which it forms part, that is, with said bundle of elements comprising a stack of the laminations.
Preferably, the core element bias means comprises a mass of compressible resilient material disposed between the housing means and the bundled elements and operable to exert a restoring force on any element displaced along its length from the quiescent pole face locus. Such element bias means may also form part of the attachment bias means in combination with a part extending between the body and housing.
The attachment means may comprise housing bias means between the body and housing and, depending upon disposition of the element bias means, part of the element bias means.